1. Field of the Invention
This invention relates to screenless completion systems in wellbores, and more particularly, to an apparatus and method using a thermosetting plastic mixed with sand which is pumped into a well formation and setting the plastic by applying heat thereto.
2. Description of the Prior Art
As part of the completion of a well after perforating a well formation, fracturing materials are typically pumped into the well and forced into the fracture formation. A common material used in fracturing is sand or another similar material. A problem exists in maintaining the sand in the fractured formation so that it does not flow out with the formation fluid. Accordingly, there have been developed many techniques for holding the sand and preventing this flow-back problem.
One common way to hold in the sand is to install a sand screen in the wellbore adjacent to the formation. Such a sand screen will allow fluid flow from the well formation but will physically keep the sand in place. Of course, this requires special equipment installed in the wellbore and therefore there is additional expense involved.
In many well formations, a mechanical screen is unnecessary or impractical. In such cases, special materials are pumped into the well with the sand during the fracturing process or after the pumping of the sand which subsequently hold the sand in place. For example, a gel material may be mixed with the sand such that the gel subsequently sets, after fracturing the formation, to form a solid gel containing sand in the fractured formation. This essentially bonds the sand together so that it will not flow back out of the formation but will still allow the flow of formation fluid therethrough. The setting of the gel may be accomplished by mixing a self-setting gel so that it sets after a certain period of time, or a setting chemical may be subsequently pumped down the wellbore which causes the gel to set.
In some cases, however, a self-setting gel is undesirable because the operator may not know in advance the time at which it will be desirable for the gel to set. Also, it may be impractical or undesirable to use a separate chemical pumped down the wellbore. The present invention solves these problems by providing a method of screenless well completion which utilizes a thermosetting plastic mixed with the sand. When the sand and plastic are pumped down into the formation to fracture it, the plastic remains in a substantially liquid state. At the time at which the operator wishes to set the plastic, a downhole heater is run into the well which applies heat adjacent to the formation above the setting temperature of the thermosetting plastic. The heating apparatus of the present invention may be used in this method.
The present invention includes a screenless completion system for wells which utilizes a thermosetting plastic material mixed with sand for fracturing a formation and then subsequently applying heat thereto. More specifically, the method comprises the steps of pumping a mixture of sand and thermosetting plastic material into a perforated formation, and applying heat to the formation, thereby setting the plastic so that the plastic and sand are prevented from flowing back into the well. The step of applying heat comprises pumping fuel and oxygen down the well, mixing the fuel and oxygen to form a combustible mixture, and igniting the mixture so that hot combustion gases are forced into the formation. The heater of this invention is used to apply this heat.
The oxygen will generally be provided simply by pumping air down the tool string to the heater. The supply of this air may be ambient air at the surface or air from a pressurized air tank. The fuel is pumped from a fuel tank into the tool string to the heater. A preferred fuel is a liquefied petroleum gas, such as propane, but the invention is not intended to be limited to this alone.
The method may further comprise pumping an inert gas down the well to dampen the heat for preventing damage to a casing surface adjacent to the heater. Preferably, but not by way of limitation, the inert gas is pumped through a well annulus defined between the heater and the casing. The combustion gases are mixed with the insert gas so that the hot combustion gases and heated inert gas are forced into the formation together.
One preferred inert gas is nitrogen, but other non-combustible gases could also be used. For many thermosetting plastics, the inert gas is used so that approximately 600xc2x0 F. is actually applied to the plastic material. This temperature is sufficient to cause setting of the plastic while preventing damage thereto.
The heating apparatus of the present invention is adapted for applying heat in a well and comprises a length of dual string coiled tubing having a length of first or outer tubing and a length of relatively smaller second or inner tubing disposed in the outer tubing such that a tubing annulus is defined there between, and a burner chamber in communication with said inner and outer tubing. When fuel is pumped down one of the inner and outer tubing, and oxygen is pumped down the other of the inner and outer tubing, a fuel and oxygen mixture is formed in the burner chamber. The apparatus further comprises ignition means for igniting the mixture.
Preferably, the outer tubing is adapted for connection to an oxygen supply, and the inner tubing is adapted for connection to a fuel supply. A fuel nozzle may be provided in communication with the inner tubing. The combustion chamber preferably comprises a ceramic cylinder defining a plurality of holes therein.
In one embodiment, the ignition means is mechanical and comprises a ferrous element disposed in the burner chamber, a grinder in contact with the ferrous element, and means for moving the grinder against the ferrous element for generating a spark. In one specifically preferred embodiment, the grinder is a grinding wheel attached to a rotatable shaft of a motor, and the motor is characterized by a progressive cavity motor connected to the shaft and thereby rotating the grinding wheel when a fluid is pumped through the motor. The fluid through the motor may be the fuel which is pumped down the inner tubing string.
In another embodiment, the ignition means is electronic. For example, a spark plug may be positioned adjacent to the burner chamber and power provided therethrough from a wire to the surface or other means of providing an electrical connection so that a spark is generated. Thus, the step of igniting in the previously described method may comprise creating a spark by rotating a grinding wheel against a ferrous element adjacent to the fuel and oxygen mixture or by providing electrical power to a spark plug adjacent to the mixture.
Numerous objects and advantages of the invention will become apparent as the following detailed description of the preferred embodiments is read in conjunction with the drawings which illustrate such embodiments.